Generally, the distal end of a guidewire is introduced into a body by a physician, e.g., through a puncture opening. The physician manipulates the tip of the guidewire through tortuous aspects of the body's passageways to a site to be treated. A catheter or other medical device is advanced over the guidewire to the treatment site and the guidewire is then removed, leaving the catheter in place.
In order for the physician to have maximal control over the guidewire, and to ensure the patient's safety, it is important that the guidewire be as small in diameter as possible, particularly in the tip region, but not so small as to create a danger of the tip breaking loose in the body. It is also important that the guidewire be smooth to allow ready advancement and retraction within the passageways; that the distal tip of the guidewire be highly flexible to permit negotiation of difficult turns within the body; that the guidewire be stiff enough axially to be advanced by thrust from the proximal end outside the body without kinking, i.e., turning back upon itself; and that the guidewire have good steerability or torque response. Most prior art guidewires compromise these desired features.
To aid the steering of the element into a desired lumen, it is common to give at least the tip portion of the element a predetermined curvature that complements the lumen path. Many guidewires available today have a curvature formed in the factory during manufacture. Some guidewires made of conventional materials, e.g., stainless steel, can be tip-formed by the physician prior to insertion, a feature found desirable by many.
Fuji Terumo Co. Ltd., EPA 0 141 006 describes a guidewire having at least portions of the body and/or flexible distal end formed of a superelastic metal member, e.g., a specially heat treated Ti--Ni alloy (Nitinol). The end may have a curved tip to aid steering. Because of the high elongation of the superelastic materials, a guidewire which has been previously curvedly deformed can be straightened when being introduced to the body through a needle and then will restore itself to the original curved shape when inserted in a blood vessel (p. 15).
In WO 88/094940, also by Terumo, a guidewire is formed by differentially heat treating a linear material, e.g., an elastic alloy such as an ultra elastic alloy (e.g., Ni--Ti) so that the flexibility increases progressively from the base to the tip.